Apparatus and circuits with dual polarization transistors and methods of fabricating the same

ABSTRACT

Apparatus and circuits with dual polarization transistors and methods of fabricating the same are disclosed. In one example, a semiconductor structure is disclosed. The semiconductor structure includes: a substrate; an active layer that is formed over the substrate and comprises a first active portion having a first thickness and a second active portion having a second thickness; a first transistor comprising a first source region, a first drain region, and a first gate structure formed over the first active portion and between the first source region and the first drain region; and a second transistor comprising a second source region, a second drain region, and a second gate structure formed over the second active portion and between the second source region and the second drain region, wherein the first thickness is different from the second thickness.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/576,554, filed Sep. 19, 2019, which claims priority to U.S.Provisional Patent Application No. 62/753,500, entitled “APPARATUS WITHDUAL POLARIZATION TRANSISTORS AND METHODS OF FABRICATING THE SAME,” andfiled on Oct. 31, 2018, the entireties of each are incorporated byreference herein.

BACKGROUND

In an integrated circuit (IC), an enhancement-mode N-type transistor,e.g. enhancement-mode high-electron-mobility transistor (E-HEMT), may beused as a pull-up device to minimize static current. In order to achievenear full-rail pull-up voltage and fast slew rate, a significantly largeover-drive voltage is needed for an N-Type enhancement-mode transistor.That is, the voltage difference between gate and source (Vgs) should bemuch larger than the threshold voltage (Vt), i.e. (Vgs−Vt>>0). It isimperative to use a multi-stage E-HEMT based driver for integratedcircuit to minimize static current. Nevertheless, multi-stage E-HEMTbased drivers will not have enough over-drive voltage (especially forthe last-stage driver) due to one Vt drop across each stage of E-HEMTpull-up device and one forward voltage (Vf) drop across boot-strapdiode. Although one can reduce the Vt for the pull-up E-HEMT transistorsand Vf of diode-connected E-HEMT rectifier of multi-stage drivers toprovide significantly enough over-drive voltage and dramatically reducestatic current, the noise immunity will be compromised.

In an existing semiconductor wafer, transistors formed on the wafer haveidentical structure such that they have a same threshold voltage Vt.When Vt of one transistor is reduced, Vt's of other transistors on thewafer are reduced accordingly. As Vt being reduced in this case, a powerswitch HEMT driven by the HEMT-based driver will have a poor noiseimmunity because the power switch HEMT cannot withstand a largeback-feed-through impulse voltage to its gate. Thus, existing apparatusand circuits including multiple transistors are not entirelysatisfactory.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that various features are not necessarily drawn to scale. In fact,the dimensions and geometries of the various features may be arbitrarilyincreased or reduced for clarity of discussion. Like reference numeralsdenote like features throughout specification and drawings.

FIG. 1 illustrates an exemplary circuit having a multi-stageboot-strapped driver, in accordance with some embodiments of the presentdisclosure.

FIG. 2 illustrates a cross-sectional view of an exemplary semiconductordevice including dual polarization transistors, in accordance with someembodiments of the present disclosure.

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 3I, 3J, 3K, 3L, 3M, 3N and 3Oillustrate cross-sectional views of an exemplary semiconductor deviceduring various fabrication stages, in accordance with some embodimentsof the present disclosure.

FIG. 4A and FIG. 4B show a flow chart illustrating an exemplary methodfor forming a semiconductor device including dual polarizationtransistors, in accordance with some embodiments of the presentdisclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure describes various exemplary embodiments forimplementing different features of the subject matter. Specific examplesof components and arrangements are described below to simplify thepresent disclosure. These are, of course, merely examples and are notintended to be limiting. For example, the formation of a first featureover or on a second feature in the description that follows may includeembodiments in which the first and second features are formed in directcontact, and may also include embodiments in which additional featuresmay be formed between the first and second features, such that the firstand second features may not be in direct contact. In addition, thepresent disclosure may repeat reference numerals and/or letters in thevarious examples. This repetition is for the purpose of simplicity andclarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. Terms such as“attached,” “affixed,” “connected” and “interconnected,” refer to arelationship wherein structures are secured or attached to one anothereither directly or indirectly through intervening structures, as well asboth movable or rigid attachments or relationships, unless expresslydescribed otherwise.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Reference will now be made in detail to the present embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

An enhancement-mode high-electron-mobility transistor (HEMT), e.g. agallium nitride (GaN) HEMT, has superior characteristics to enable highperformance and smaller form factor in power conversion and radiofrequency power amplifier and power switch applications compared tosilicon based transistors. But there is no viable p-type HEMT availablemostly due to much lower p-type mobility and partly due to twodimensional hole gas (2DHG) band structure. While n-type GaN HEMTs areused in an integrated circuit, to minimize static current, the pull-updevices are mostly based on enhancement-mode n-type transistors ratherthan depletion-mode n-type transistors.

A multi-stage HEMT based driver can be used for an integrated circuit tominimize static current. But multi-stage HEMT based drivers will nothave enough over-drive voltage (especially for the last-stage driver)due to one threshold voltage (Vt) drop across each stage of HEMT pull-updevice and one forward voltage (Vt) drop across boot-strap diode.Although one can reduce the Vt for the pull-up HEMT transistors and Vfof diode-connected HEMT rectifier of multi-stage drivers to providesignificantly enough over-drive voltage and dramatically reduce staticcurrent, the noise immunity will be compromised.

Instead of reducing a single value of the threshold voltage (Vt) of theHEMT transistors in an IC, the present teaching discloses apparatus andcircuits including dual-Vt transistors and their fabrication process. Inone embodiment, two transistors formed on a same wafer have differentVt's. In particular, two transistors have different active layerthicknesses to obtain different Vt's from each other. The two activelayers may be aluminum gallium nitride (AlGaN) layers on a same GaNlayer, which is a channel layer for the transistors. Differentthicknesses of the AlGaN layers can change the amount of spontaneouspolarization and piezoelectric polarization between the AlGaN layer andthe GaN layer. A thicker AlGaN layer introduces higher polarizations andhence creates more amount of two dimensional electric gas (2-DEG) tolower the Vt. Hence, GaN devices having different Vt's can beimplemented by depositing AlGaN layers with different thicknesses byepitaxial growth. In an exemplary method of fabricating the dual-Vttransistors, the two AlGaN layers can be formed with differentthicknesses on a same channel layer.

The disclosed apparatus can adjust the polarization between the AlGaNlayer and the GaN layer by depositing AlGaN layers with differentthicknesses to create dual-Vt (or various-Vt) transistors on a samesemiconductor wafer; and generate different amount of 2-DEG fortransistors at different locations of the same wafer.

The present disclosure is applicable to any transistor based IC. Theproposed apparatus and methods can enable a transistor based IC toreduce the static current significantly and have significantly largeover-drive voltages for drivers of concern; without compromising noiseimmunity while increasing over-drive voltages and reducing staticcurrents. In addition, the disclosed apparatus and methods can provideIC designers the flexibility of using different Vt devices for specificfunctions of improving performance, reducing static current, improvingnoise immunity, etc.

FIG. 1 illustrates an exemplary circuit 100 having a multi-stageboot-strapped driver, in accordance with some embodiments of the presentdisclosure. As shown in FIG. 1, the circuit 100 includes a driver havingmultiple stages 110, 120, 130 serially connected to drive a power switchHEMT 175. Each stage includes multiple transistors.

The stage 110 in this example includes transistors 141, 151, 152, 153,154, 155, 156. In one embodiment, among these transistors, thetransistor 154 is a low voltage depletion-mode high electron mobilitytransistor (LV D-HEMT) 192; while each of the other transistors 141,151, 152, 153, 155, 156 is a low voltage enhancement-mode high electronmobility transistor (LV E-HEMT) 191.

As shown in FIG. 1, the gate of the transistor 151 is electricallyconnected to an input pin 131 of the circuit 100. The input pin 131 hasan input voltage Vin ranged from a low logic state voltage (e.g. 0V) toa high logic state voltage (e.g. 6V). When the circuit 100 is turnedoff, the Vin is 0. The circuit 100 is turned on after the Vin isincreased to 6V. The transistor 151 has a source electrically connectedto ground Vss 111 which has a ground voltage 0V; and has a drainelectrically connected to a source of the transistor 154. The transistor152 in this example has a gate electrically connected to the input pin131, a source electrically connected to the ground Vss 111 which has aground voltage 0V, and a drain electrically connected to a source of thetransistor 155. Similarly, the transistor 153 in this example has a gateelectrically connected to the input pin 131, a source electricallyconnected to the ground Vss 111 which has a ground voltage 0V, and adrain electrically connected to a source of the transistor 156.

The transistor 154 in this example has a gate electrically connected toits own source, which is electrically connected to the drain of thetransistor 151. Drain of the transistor 154 is electrically connected toa source of the transistor 141. The transistor 155 in this example has agate electrically connected to the source of the transistor 154 andelectrically connected to the drain of the transistor 151. Thetransistor 155 has a source electrically connected to the drain of thetransistor 152, and a drain electrically connected to a power supply pinVDD 101 which has a positive power supply voltage (e.g. 6V). Similarly,the transistor 156 in this example has a gate electrically connected tothe source of the transistor 154 and electrically connected to the drainof the transistor 151, a source electrically connected to the drain ofthe transistor 153, and a drain electrically connected to the powersupply pin VDD 101 which has a positive power supply voltage 6V.

The transistor 141 in this example has a gate electrically connected toits own drain, which is electrically connected to the power supply pinVDD 101 which has a positive power supply voltage 6V. The transistor 141connected in this specific configuration is functioning like a rectifieror diode and is conventionally called as a diode-connected transistor.Source of the transistor 141 is electrically connected to the drain ofthe transistor 154. The stage 110 further includes a capacitor 121coupled between the source of the transistor 141 and the source of thetransistor 155.

The stage 120 in this example includes transistors 142, 161, 162, 163,164, 165, 166. In one embodiment, among these transistors, thetransistor 164 is a low voltage depletion-mode high electron mobilitytransistor (LV D-HEMT) 192; while each of the other transistors 142,161, 162, 163, 165, 166 is a low voltage enhancement-mode high electronmobility transistor (LV E-HEMT) 191.

As shown in FIG. 1, the gate of the transistor 161 is electricallyconnected to a node 181, which is electrically connected to the sourceof the transistor 156 and the drain of the transistor 153. The node 181has a voltage ranged between Vss and VDD (0 and 6V). When the circuit100 is turned off, the Vin is 0, such that the transistor 153 is turnedoff and the transistor 156 is turned on. The node 181 has the samevoltage 6V as the power supply pin VDD 101. When the circuit 100 isturned on and the Vin has a voltage of 6V, the transistor 153 is turnedon and the transistor 156 is turned off. The node 181 has the samevoltage 0V as the ground Vss 111.

The transistor 161 has a source electrically connected to ground Vss 111which has a ground voltage 0V; and has a drain electrically connected toa source of the transistor 164. The transistor 162 in this example has agate electrically connected to the node 181, a source electricallyconnected to the ground Vss 111 which has a ground voltage 0V, and adrain electrically connected to a source of the transistor 165.Similarly, the transistor 163 in this example has a gate electricallyconnected to the node 181, a source electrically connected to the groundVss 111 which has a ground voltage 0V, and a drain electricallyconnected to a source of the transistor 166.

The transistor 164 in this example has a gate electrically connected toits own source, which is electrically connected to the drain of thetransistor 161. Drain of the transistor 164 is electrically connected toa source of the transistor 142. The transistor 165 in this example has agate electrically connected to a node 185, which is electricallyconnected to the source of the transistor 164 and electrically connectedto the drain of the transistor 161. The transistor 165 has a sourceelectrically connected to the drain of the transistor 162, and a drainelectrically connected to the source of the transistor 142. Thetransistor 166 in this example has a gate electrically connected to anode 186, which is electrically connected to the source of thetransistor 165 and electrically connected to the drain of the transistor162, a source electrically connected to the drain of the transistor 163,and a drain electrically connected to a power supply pin VDD 102 whichhas a positive power supply voltage (e.g. 6V).

The transistor 142 in this example has a gate electrically connected toits own drain (i.e. diode-connected to act like a rectifier or diode),which is electrically connected to the power supply pin VDD 102 whichhas a positive power supply voltage 6V. Source of the transistor 142 iselectrically connected to the drain of the transistor 164 and the drainof the transistor 165. The stage 120 further includes a capacitor 122coupled between a node 184 electrically connected to the source of thetransistor 142 and a node 183 electrically connected to the source ofthe transistor 166.

The stage 130 in this example includes transistors 143, 171, 172, 173,174. In one embodiment, each of these transistors is a low voltageenhancement-mode high electron mobility transistor (LV E-HEMT) 191. Asshown in FIG. 1, the gate of the transistor 171 is electricallyconnected to a node 182, which is electrically connected to the node181, the source of the transistor 156 and the drain of the transistor153. Same as the node 181, the node 182 has a voltage ranged between Vssand VDD (0 and 6V). When the circuit 100 is turned off, the Vin is 0,such that the transistor 153 is turned off and the transistor 156 isturned on. The node 181 and the node 182 have the same voltage 6V as thepower supply pin VDD 101. When the circuit 100 is turned on and the Vinhas a voltage of 6V, the transistor 153 is turned on and the transistor156 is turned off. The node 181 and the node 182 have the same voltage0V as the ground Vss 111.

The transistor 171 has a source electrically connected to ground Vss 111which has a ground voltage 0V; and has a drain electrically connected toa source of the transistor 173. The transistor 172 in this example has agate electrically connected to the node 182, a source electricallyconnected to the ground Vss 111 which has a ground voltage 0V, and adrain electrically connected to a source of the transistor 174.

The transistor 173 in this example has a gate electrically connected tothe node 186, which is electrically connected to the source of thetransistor 165. The transistor 173 has a source electrically connectedto the drain of the transistor 171, and a drain electrically connectedto a source of the transistor 143. The transistor 174 in this examplehas a gate electrically connected to a node 187, which is electricallyconnected to the source of the transistor 173 and electrically connectedto the drain of the transistor 171. The transistor 174 has a sourceelectrically connected to the drain of the transistor 172, and a drainelectrically connected to a power supply pin VDD 103 which has apositive power supply voltage (e.g. 6V).

The transistor 143 in this example has a gate electrically connected toits own drain (i.e. diode-connected to act like a rectifier or diode),which is electrically connected to the power supply pin VDD 103 whichhas a positive power supply voltage 6V. Source of the transistor 143 iselectrically connected to the drain of the transistor 173. The stage 130further includes a capacitor 123 coupled between a node 189 electricallyconnected to the source of the transistor 143 and a node 188electrically connected to the source of the transistor 174.

As such, the stages 110, 120, 130 are serially connected to form amulti-stage driver that drives a power switch transistor 175. In oneembodiment, the power switch HEMT 175 is a high voltage enhancement-modehigh electron mobility transistor (HV E-HEMT) 193. As shown in FIG. 1,the power switch HEMT 175 has a gate electrically connected to the node188, a source electrically connected to ground Vss 112 which has aground voltage 0V, and a drain electrically connected to an output pin133 of the circuit 100. In some embodiments, the circuit 100 can serveas a low-side driver in a half-bridge or full-bridge power converter,where the output pin 133 serves as a low-side voltage output (LoVout).

Most transistors in FIG. 1 are enhancement-mode N-type transistors. Thatis, the circuit 100 uses mostly enhancement-mode N-type transistors aspull-up devices to minimize static current. In order to achieve nearfull-rail pull-up voltage and fast slew rate, a significantly largeover-drive voltage is needed for the N-Type enhancement-mode transistor.That is, the voltage difference between gate and source (Vgs) should bemuch larger than the threshold voltage (Vt), i.e. (Vgs−Vt>>0). While themulti-stage driver of the circuit 100 can minimize static current, eachstage of E-HEMT pull-up device consumes at least one Vt voltage drop.

As discussed above, the node 181 has a voltage ranged between Vss andVDD (0 and 6V). When the circuit 100 is turned off, the Vin is 0, suchthat the transistor 153 is turned off and the transistor 156 is turnedon. The node 181 has the same voltage 6V as the power supply pin VDD101, which enables the transistors 161, 162, 163 to be turned on. Assuch, the node 185 is electrically connected to the ground Vss 111, andhas a voltage close to 0V. As such, the transistor 165 is turned off,and the node 186 is electrically connected to the ground Vss 111 and hasa voltage 0V. Accordingly, the transistor 166 is turned off, and thenode 183 is electrically connected to the ground Vss 111 and has avoltage 0V. In this case, the capacitor 122 is charged by the powersupply pin VDD 102 via the transistor 142. In this example, thetransistor 142 is a diode-connected HEMT used as a rectifying diode,which naturally has a forward voltage (Vf). That is, the voltage at thenode 184 will maximally be charged to 6V−Vf. In a first example,assuming the forward voltages and threshold voltages of all transistorsin FIG. 1 are equal to 1.5V, the maximum voltage at the node 184 whenthe circuit 100 is turned off is 6V−1.5V=4.5V.

When the circuit 100 is turned on and the Vin has a voltage of 6V, thetransistor 153 is turned on and the transistor 156 is turned off. Thenode 181 has the same voltage 0V as the ground Vss 111, which enablesthe transistors 161, 162, 163 to be turned off As such, the node 185 iselectrically connected to the node 184, and has a same voltage as thenode 184. This induces the transistor 165 to be turned on, which enablesthe node 186 to be charged by the voltage at the node 184. This in turninduces the transistor 166 to be turned on, which enables the node 183to be charged by the power supply pin VDD 102. As such, the voltage atthe node 183 can maximally be charged to 6V, same as the voltage of thepower supply pin VDD 102. Based on the 4.5V voltage difference stored bythe capacitor 122 when the circuit 100 is off, the voltage at the node184 can maximally be charged and increased to 6V+4.5V=10.5V, i.e. thevoltage at the node 184 is boot-strapped to 10.5V. Accordingly, the node185, which is electrically connected to both the source and the gate ofthe transistor 164, is charged to 10.5V as well.

While the node 186 is also charged by the voltage 10.5V at the node 184,the voltage of the node 186 cannot reach 10.5V. Because the node 186 iselectrically connected to the source of the transistor 165, to keep thetransistor 165 on, the gate source voltage difference Vgs of thetransistor 165 must be larger than the threshold voltage (Vt) of thetransistor 165. As it is assumed Vt=1.5V in the first example, themaximum voltage the node 186 can reach in the first example when thecircuit 100 is turned on is 10.5V−Vt=10.5V−1.5V=9V. As such, anenhancement-mode high-electron-mobility transistor (E-HEMT) pull-updevice consumes at least one Vt voltage drop.

The node 182 is electrically connected to the node 181 and has a samevoltage as that of the node 181. That is, when the circuit 100 is turnedoff, the node 182 has the voltage 6V; when the circuit 100 is turned on,the node 182 has the voltage 0V. When the circuit 100 is turned off, the6V voltage at the node 182 enables the transistors 171, 172 to be turnedon. As such, the node 187 is electrically connected to the ground Vss111, and has a voltage 0V. Here, the transistor 173 is turned off due tothe 0V voltage at the node 186 when the circuit 100 is turned off asdiscussed above. Because the node 187 has the voltage 0V, the transistor174 is turned off, and the node 188 is electrically connected to theground Vss 111 and has a voltage 0V. In this case, the capacitor 123 ischarged by the power supply pin VDD 103 via the transistor 143. In thisexample, the transistor 143 is a diode-connected HEMT used as arectifying diode, which naturally has a forward voltage (Vf). That is,the voltage at the node 189 will maximally be charged to 6V−Vf. In thefirst example, assuming the forward voltages and threshold voltages ofall transistors in FIG. 1 are equal to 1.5V, the maximum voltage at thenode 189 when the circuit 100 is turned off is 6V−1.5V=4.5V.

When the circuit 100 is turned on, the node 182, like the node 181, hasthe same voltage 0V as the ground Vss 111, which enables the transistors171, 172 to be turned off. As discussed above, the node 186, which iselectrically connected to the gate of the transistor 173, has a maximumvoltage of 9V when the circuit 100 is turned on. As such, the transistor173 is turned on and the node 187 is charged by the node 189. Thisinduces the transistor 174 to be turned on, which enables the node 188to be charged by the power supply pin VDD 103. As such, the voltage atthe node 188 can maximally be charged to 6V, same as the voltage of thepower supply pin VDD 102. Based on the 4.5V voltage difference stored bythe capacitor 123 when the circuit 100 is off, the voltage at the node189 can maximally be charged and increased to 6V+4.5V=10.5V, i.e. thevoltage at the node 189 is boot-strapped to 10.5V.

While the node 187 is charged by the voltage 10.5V at the node 189, thevoltage of the node 187 cannot reach 10.5V. Because the node 187 iselectrically connected to the source of the transistor 173, to keep thetransistor 173 on, the gate source voltage difference Vgs of thetransistor 173 must be larger than the threshold voltage (Vt) of thetransistor 173. The gate of the transistor 173 is electrically connectedto the node 186, which has a maximum voltage 9V when the circuit 100 isturned on. As it is assumed Vt=1.5V in the first example, the maximumvoltage the node 187 can reach in the first example when the circuit 100is turned on is 9V−Vt=9V−1.5V=7.5V. Now the transistor 174 has a gatesource voltage difference Vgs=7.5V−6V=1.5V, which is exactly equal tothe threshold voltage Vt=1.5V of the transistor 174. This leaves novoltage margin at the last stage of the multi-stage boot-strappeddriver. That is, in the first example where Vf=Vt=1.5V, there is notenough over-drive voltage to drive the power switch HEMT 175. Even ifthe power switch HEMT 175 can be driven, it would be significantly slowas the current flowing through the transistor 174 and the node 188 wouldbe very slow due to no Vgs margin compared to the Vt. The aboveconclusion has not even taken into consideration of the Vt variation(e.g. 3-σ variation of 0.5V), which typically exists in all processtechnologies. After counting the 3-σ variation of 0.5V, the circuit 100,under the Vt=1.5V assumption, may not be able to drive the power switchHEMT 175 at all.

In a second example, it is assumed the forward voltages and thresholdvoltages of all transistors in FIG. 1 are equal to 1V. In this case,when the circuit 100 is turned off, the node 181 has the same voltage6V, which enables the transistors 161, 162, 163 to be turned on. Assuch, the node 185 is electrically connected to the ground Vss 111 andhas a voltage 0V. As such, the transistor 165 is turned off, and thenode 186 is electrically connected to the ground Vss 111 and has avoltage 0V. Accordingly, the transistor 166 is turned off, and the node183 is electrically connected to the ground Vss 111 and has a voltage0V. The capacitor 122 is charged by the power supply pin VDD 102 via thetransistor 142. Because the transistor 142 is a diode-connected HEMTused as a rectifying diode which naturally has a forward voltage (Vt),the node 184 can have a maximum voltage of 6V−Vf=6V−1V=5V.

When the circuit 100 is turned on, the node 181 has the same voltage 0Vas the ground Vss 111, which enables the transistors 161, 162, 163 to beturned off. As such, the node 185 is electrically connected to the node184, and has a same voltage as the node 184. This induces the transistor165 to be turned on, which enables the node 186 to be charged by thevoltage at the node 184. This in turn induces the transistor 166 to beturned on, which enables the node 183 to be charged by the power supplypin VDD 102. As such, the node 183 has a maximum voltage of 6V, same asthe voltage of the power supply pin VDD 102. Based on the 5V voltagedifference stored by the capacitor 122 when the circuit 100 is off, thevoltage at the node 184 can maximally be charged and increased to6V+5V=11V, i.e. the voltage at the node 184 is boot-strapped to 11V.Accordingly, the node 185, which is electrically connected to both thesource and the gate of the transistor 164, is charged to 11V as well.While the node 186 is also charged by the voltage 11V at the node 184,the voltage of the node 186 cannot reach 11V. Because the node 186 iselectrically connected to the source of the transistor 165, to keep thetransistor 165 on, the gate source voltage difference Vgs of thetransistor 165 must be larger than the threshold voltage (Vt) of thetransistor 165. As it is assumed Vt=1V in the second example, themaximum voltage the node 186 can reach in the second example when thecircuit 100 is turned on is 11V−Vt=11V−1V=10V.

The node 182 is electrically connected to the node 181 and has a samevoltage as that of the node 181. That is, when the circuit 100 is turnedoff, the node 182 has the voltage 6V; when the circuit 100 is turned on,the node 182 has the voltage 0V. When the circuit 100 is turned off, the6V voltage at the node 182 enables the transistors 171, 172 to be turnedon. As such, the node 187 is electrically connected to the ground Vss111, and has a voltage 0V. Here, the transistor 173 is turned off due tothe 0V voltage at the node 186 when the circuit 100 is turned off asdiscussed above. Because the node 187 has the voltage 0V, the transistor174 is turned off, and the node 188 is electrically connected to theground Vss 111 and has a voltage 0V. In this case, the capacitor 123 ischarged by the power supply pin VDD 103 via the transistor 143. Becausethe transistor 143 is a diode-connected HEMT used as a rectifying diodewhich naturally has a forward voltage (Vf), the node 189 has a maximumvoltage of 6V−Vf=6V−1V=5V.

When the circuit 100 is turned on, the node 182, like the node 181, hasthe same voltage 0V as the ground Vss 111, which enables the transistors171, 172 to be turned off. As discussed above, the node 186, which iselectrically connected to the gate of the transistor 173, has a maximumvoltage of 10V when the circuit 100 is turned on. As such, thetransistor 173 is turned on and the node 187 is charged by the node 189.This induces the transistor 174 to be turned on, which enables the node188 to be charged by the power supply pin VDD 103. As such, the voltageat the node 188 can maximally be charged to 6V, same as the voltage ofthe power supply pin VDD 102. Based on the 5V voltage difference storedby the capacitor 123 when the circuit 100 is off, the voltage at thenode 189 can maximally be charged and increased to 6V+5V=11V, i.e. thevoltage at the node 189 is boot-strapped to 11V.

While the node 187 is charged by the voltage 11V at the node 189, thevoltage of the node 187 cannot reach 11V. Because the node 187 iselectrically connected to the source of the transistor 173, to keep thetransistor 173 on, the gate source voltage difference Vgs of thetransistor 173 must be larger than the threshold voltage (Vt) of thetransistor 173. The gate of the transistor 173 is electrically connectedto the node 186, which has a maximum voltage 10V when the circuit 100 isturned on. As it is assumed Vt=1V in the second example, the maximumvoltage the node 187 can reach in the second example when the circuit100 is turned on is 10V−Vt=10V−1V=9V. Now the transistor 174 has a gatesource voltage difference Vgs=9V−6V=3V, which is much larger than thethreshold voltage Vt=1V of the transistor 174. This leaves enoughvoltage margin at the last stage of the multi-stage boot-strappeddriver. That is, in the second example where Vf=Vt=1V, there is enoughover-drive voltage to drive the power switch HEMT 175. However, sinceall transistors, including the power switch HEMT 175, in FIG. 1 areusing a same Vt, a reduced Vt at the power switch HEMT 175 may cause thenoise immunity of the output power switch 175 become significantly worsedue to not being able to withstand a large back-feed-through impulse(di/dt) voltage to the gate of the output power switch 175. Becausethere is inevitable parasitic capacitance between the drain and the gateof the power switch HEMT 175, a voltage impulse will feed back from thedrain of the power switch HEMT 175 to the gate of the power switch HEMT175 through the parasitic capacitance. This could accidently turn on thepower switch HEMT 175 so long as the noise voltage is larger than thereduced Vt of the power switch HEMT 175, even when the circuit 100 isturned off.

As such, in a third example, the forward voltages and threshold voltagesof all transistors in FIG. 1 are not all the same. In the third example,it is assumed that the transistors 142, 143, 165, 166, 173, 174 have asmaller Vt of 1V, while the other transistors in FIG. 1 have a larger Vtof 1.5V. In this case, when the circuit 100 is turned off, the node 181has the same voltage 6V, which enables the transistors 161, 162, 163 tobe turned on. As such, the node 185 is electrically connected to theground Vss 111 and has a voltage 0V. As such, the transistor 165 isturned off, and the node 186 is electrically connected to the ground Vss111 and has a voltage 0V. Accordingly, the transistor 166 is turned off,and the node 183 is electrically connected to the ground Vss 111 and hasa voltage 0V. The capacitor 122 is charged by the power supply pin VDD102 via the transistor 142. Because the transistor 142 has a forwardvoltage Vf equal to its Vt, the node 184 can have a maximum voltage of6V−Vf=6V−1V=5V.

When the circuit 100 is turned on, the node 181 has the same voltage 0Vas the ground Vss 111, which enables the transistors 161, 162, 163 to beturned off. As such, the node 185 is electrically connected to the node184, and has a same voltage as the node 184. This induces the transistor165 to be turned on, which enables the node 186 to be charged by thevoltage at the node 184. This in turn induces the transistor 166 to beturned on, which enables the node 183 to be charged by the power supplypin VDD 102. As such, the node 183 has a maximum voltage of 6V, same asthe voltage of the power supply pin VDD 102. Based on the 5V voltagedifference stored by the capacitor 122 when the circuit 100 is off, thevoltage at the node 184 can maximally be charged and increased to6V+5V=11V, i.e. the voltage at the node 184 is boot-strapped to 11V.Accordingly, the node 185, which is electrically connected to both thesource and the gate of the transistor 164, is charged to 11V as well.While the node 186 is also charged by the voltage 11V at the node 184,the voltage of the node 186 cannot reach 11V. Because the node 186 iselectrically connected to the source of the transistor 165, to keep thetransistor 165 on, the gate source voltage difference Vgs of thetransistor 165 must be larger than the Vt=1V of the transistor 165. Sothe maximum voltage the node 186 can reach in the third example when thecircuit 100 is turned on is 11V−1V=10V.

The node 182 is electrically connected to the node 181 and has a samevoltage as that of the node 181. That is, when the circuit 100 is turnedoff, the node 182 has the voltage 6V; when the circuit 100 is turned on,the node 182 has the voltage 0V. When the circuit 100 is turned off, the6V voltage at the node 182 enables the transistors 171, 172 to be turnedon. As such, the node 187 is electrically connected to the ground Vss111, and has a voltage 0V. Here, the transistor 173 is turned off due tothe 0V voltage at the node 186 when the circuit 100 is turned off asdiscussed above. Because the node 187 has the voltage 0V, the transistor174 is turned off, and the node 188 is electrically connected to theground Vss 111 and has a voltage 0V. In this case, the capacitor 123 ischarged by the power supply pin VDD 103 via the diode-connectedtransistor 143. Because the diode-connected transistor 143 has a forwardvoltage Vf equal to its Vt, the node 189 has a maximum voltage of6V−Vf=6V−1V=5V.

When the circuit 100 is turned on, the node 182, like the node 181, hasthe same voltage 0V as the ground Vss 111, which enables the transistors171, 172 to be turned off. As discussed above, the node 186, which iselectrically connected to the gate of the transistor 173, has a maximumvoltage of 10V when the circuit 100 is turned on. As such, thetransistor 173 is turned on and the node 187 is charged by the node 189.This induces the transistor 174 to be turned on, which enables the node188 to be charged by the power supply pin VDD 103. As such, the voltageat the node 188 can maximally be charged to 6V, same as the voltage ofthe power supply pin VDD 102. Based on the 5V voltage difference storedby the capacitor 123 when the circuit 100 is off, the voltage at thenode 189 can maximally be charged and increased to 6V+5V=11V, i.e. thevoltage at the node 189 is boot-strapped to 11V.

While the node 187 is charged by the voltage 11V at the node 189, thevoltage of the node 187 cannot reach 11V. Because the node 187 iselectrically connected to the source of the transistor 173, to keep thetransistor 173 on, the gate source voltage difference Vgs of thetransistor 173 must be larger than the threshold voltage Vt=1V of thetransistor 173. Because the gate of the transistor 173 is electricallyconnected to the node 186, which has a maximum voltage 10V when thecircuit 100 is turned on, the maximum voltage the node 187 can reach inthe third example when the circuit 100 is turned on is 10V−Vt=10V−1V=9V.Now the transistor 174 has a gate source voltage differenceVgs=9V−6V=3V, which is much larger than the threshold voltage Vt=1V ofthe transistor 174. This leaves enough voltage margin at the last stageof the multi-stage boot-strapped driver. That is, in the third examplewhere the transistors 142, 143, 165, 166, 173, 174 have a smaller Vt=1V,there is enough over-drive voltage to drive the power switch HEMT 175.In addition, since all other transistors, including the power switchHEMT 175, in FIG. 1 are having a larger Vt=1.5V, the noise immunity ofthe output power switch 175 will be better than the second example,because a larger Vt of the power switch HEMT 175 can significantlywithstand impulse voltage noise fed back from the drain of the powerswitch HEMT 175 to the gate of the power switch HEMT 175.

In various embodiments, the power switch HEMT 175 may have an evenlarger Vt like 2V. The disclosed circuit design for dual-Vt or multi-Vttransistors can reduce both Vt of the pull-up E-HEMT transistors and Vfof the diode-connected E-HEMT rectifiers of the multi-stage driver toprovide enough over-drive voltage and dramatically reduce staticcurrent, without compromising the noise immunity of the output powerswitch. To use dual-Vt or multi-Vt transistors in a same IC, differentactive layer thicknesses can be used for different transistors formed ona same wafer. While a thicker active layer, e.g. a thicker AlGaN layer,introduces higher polarizations and more amount of 2-DEG to be used forimplementing a low-Vt transistor; a thinner active layer introduceslower polarizations and less amount of 2-DEG to be used for implementinga high-Vt transistor.

FIG. 2 illustrates a cross-sectional view of an exemplary semiconductordevice 200 including dual polarization transistors, in accordance withsome embodiments of the present disclosure. As shown in FIG. 2, thesemiconductor device 200 in this example includes a silicon layer 210and a transition layer 220 disposed on the silicon layer 210. Thesemiconductor device 200 further includes a first layer 230 comprising afirst III-V semiconductor material formed over the transition layer 220.

The semiconductor device 200 further includes a second layer 240 (apolarization layer) comprising a second III-V semiconductor materialdisposed on the first layer 230. The second III-V semiconductor materialis different from the first III-V semiconductor material. For example,the first III-V semiconductor material may be gallium nitride (GaN);while the second III-V semiconductor material may be aluminum galliumnitride (AlGaN). The semiconductor device 200 further includes a firsttransistor 201 and a second transistor 202 formed over the first layer230. The second layer 240 has different thicknesses at differentlocations of the semiconductor device 200. For example, the AlGaN layer240 is thicker at the first transistor 201, and is thinner at the secondtransistor 202.

The first transistor 201 comprises a first gate structure 251, a firstsource region 281 and a first drain region 291. The second transistor202 comprises a second gate structure 252, a second source region 282and a second drain region 292. The semiconductor device 200 furtherincludes a polarization modulation layer 241, 242 disposed on the secondlayer 240, and a passivation layer 250 disposed partially on thepolarization modulation layer and partially on the second layer 240. Inone embodiment, the polarization modulation layer comprises p-type dopedGaN (pGaN).

The sources 281, 282 and the drains 291, 292 of the two transistors 201,202 are formed through the second layer 240 and the passivation layer250, and disposed on the first layer 230. The first gate structure 251is disposed on the pGaN portion 241 and between the first source region281 and the first drain region 291. The second gate structure 252 isdisposed on the pGaN portion 242 and between the second source region282 and the second drain region 292.

In one embodiment, the first transistor 201 and the second transistor202 are high electron mobility transistors to be used in a samemulti-stage driver circuit. For example, the second transistor 202 isused as a power switch transistor and has a first threshold voltage. Thefirst transistor 201 is used as a driver transistor and has a secondthreshold voltage that is lower than the first threshold voltage.Accordingly, the AlGaN layer 240 under the first transistor 201 isthicker than the AlGaN layer 240 under the second transistor 202 to havea higher polarization.

In addition, the semiconductor device 200 includes an interlayerdielectric (ILD) layer 260 disposed partially on the passivation layer250 and partially on the first transistor 201 and the second transistor202. The semiconductor device 200 also includes metal contacts 271disposed on and in contact with the sources 281, 282 and the drains 291,292 respectively, and includes a first metal layer 272 on the metalcontacts 271.

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, 3H, 3I, 3J, 3K, 3L, 3M, 3N and 3Oillustrate cross-sectional views of an exemplary semiconductor deviceduring various fabrication stages, in accordance with some embodimentsof the present disclosure. In some embodiments, the semiconductor devicemay be included in an integrated circuit (IC). In addition, FIGS. 3Athrough 3O are simplified for a better understanding of the concepts ofthe present disclosure. For example, although the figures illustrate twotransistors, it is understood the semiconductor device may include morethan two transistors, and the IC may include a number of other devicescomprising resistors, capacitors, inductors, fuses, etc., which are notshown in FIGS. 3A through 3O, for purposes of clarity of illustration.

FIG. 3A is a cross-sectional view of the semiconductor device includinga substrate 310, which is provided at one of the various stages offabrication, according to some embodiments of the present disclosure.The substrate 310 may be formed of silicon, as shown in FIG. 3A, oranother semiconductor material.

FIG. 3B is a cross-sectional view of the semiconductor device includinga transition or buffer layer 320, which is formed on the substrate 310at one of the various stages of fabrication, according to someembodiments of the present disclosure. The transition or buffer layer320 may be formed by epitaxial growth. According to various embodiments,the transition or buffer layer 320 includes a nucleation layer ofaluminum nitride (AlN) and serves as a buffer to reduce the stressbetween the substrate 310 and the layer on top of the transition orbuffer layer 320. In one embodiment, the transition or buffer layer 320and the operation step shown in FIG. 3B is optional and can be removed.

FIG. 3C is a cross-sectional view of the semiconductor device includinga first III-V semiconductor material layer 330, which is formedoptionally on the transition or buffer layer 320 or directly on thesubstrate 310 at one of the various stages of fabrication, according tosome embodiments of the present disclosure. The first III-Vsemiconductor material layer 330 may be formed by epitaxial growth.According to various embodiments, the first III-V semiconductor materiallayer 330 includes a gallium nitride (GaN). When the first III-Vsemiconductor material layer 330 is formed on the transition or bufferlayer 320, the transition or buffer layer 320 can reduce the stressbetween the substrate 310 and the first III-V semiconductor materiallayer 330. After transistors are formed over the first III-Vsemiconductor material layer 330, the first III-V semiconductor materiallayer 330 serves as a channel layer for the transistors.

FIG. 3D is a cross-sectional view of the semiconductor device includinga second III-V semiconductor material layer 331, which is formed on thefirst III-V semiconductor material layer 330 at one of the variousstages of fabrication, according to some embodiments of the presentdisclosure. The second III-V semiconductor material layer 331 may beformed by epitaxial growth. According to various embodiments, the secondIII-V semiconductor material layer 331 includes an aluminum galliumnitride (AlGaN). After transistors are formed over the first III-Vsemiconductor material layer 330 and the second III-V semiconductormaterial layer 331, a 2-dimensional electron gas (2-DEG) will be formedat the interface between the first III-V semiconductor material layer330 and the second III-V semiconductor material layer 331.

FIG. 3E is a cross-sectional view of the semiconductor device includinga third III-V semiconductor material layer 332, which is formed on aportion of the second III-V semiconductor material layer 331 at one ofthe various stages of fabrication, according to some embodiments of thepresent disclosure. The third III-V semiconductor material layer 332 maybe formed by epitaxial growth. According to various embodiments, thethird III-V semiconductor material layer 332 includes an aluminumgallium nitride (AlGaN). That is, while the second III-V semiconductormaterial layer 331 is a first AlGaN layer on the GaN layer 330, thethird III-V semiconductor material layer 332 is a second AlGaN layer onthe GaN layer 330. As shown in FIG. 3E, with a mask 333 covering theright portion of the first AlGaN layer 331, the second AlGaN layer 332is disposed on the left portion of the first AlGaN layer 331.

FIG. 3F is a cross-sectional view of the semiconductor device, where themask 333 is removed from the first AlGaN layer 331 after the secondAlGaN layer 332 is formed, at one of the various stages of fabrication,according to some embodiments of the present disclosure. After the mask333 is removed, the AlGaN layer on the GaN layer 330 has differentthicknesses at different locations of the wafer. In particular, the leftportion of the AlGaN layer (having a thickness of both the first AlGaNlayer 331 and the second AlGaN layer 332) is thicker than the rightportion of the AlGaN layer (having a thickness of merely the first AlGaNlayer 331). In some embodiments, the thickness of the right portion ofthe AlGaN layer is from about 50% to 80% of the thickness of the leftportion of the AlGaN layer.

FIG. 3G is a cross-sectional view of the semiconductor device includinga p-type doped GaN (pGaN) layer 341, 342, which is formed on the secondAlGaN layer 332 and the first AlGaN layer 331, at one of the variousstages of fabrication, according to some embodiments of the presentdisclosure. The pGaN layer 341, 342 is patterned to form island regionsshown in FIG. 3G. The patterning of the pGaN layer includes, e.g., (i)forming a masking layer (e.g., photoresist, etc.) over the pGaN layer,the masking layer including openings over the portions of the pGaN layerthat are to be removed, and (ii) removing the portions of the pGaN layerthat are left exposed by the masking layer (e.g., via a wet or dry etchprocedure). The pGaN layer 341, 342 may be called a polarizationmodulation layer, which modulates the dipole concentration in the AlGaNlayers 332, 331 to result in changing the 2-DEG concentration in theAlGaN/GaN interface channel. While the polarization modulation layer isformed for an enhancement-mode (normally off) AlGaN/GaN HEMT, thepolarization modulation layer is not needed in a depletion-mode(normally on) AlGaN/GaN HEMT.

FIG. 3H is a cross-sectional view of the semiconductor device includinga passivation layer 350, which is formed on the first AlGaN layer 331,the second AlGaN layer 332, and the polarization modulation layer at oneof the various stages of fabrication, according to some embodiments ofthe present disclosure. The passivation layer 350 is formed over thefirst AlGaN layer 331, the second AlGaN layer 332 and over the remainingportions of the polarization modulation layer 341, 342. According tovarious embodiments, the passivation layer 350 is formed using adeposition procedure (e.g., chemical deposition, physical deposition,etc.). The passivation layer 350 may comprise silicon oxide, siliconnitride, silicon oxynitride, carbon doped silicon oxide, carbon dopedsilicon nitride, carbon doped silicon oxynitride, zinc oxide, zirconiumoxide, hafnium oxide, titanium oxide, or another suitable material. Inone embodiment, after depositing the passivation layer 350, thepassivation layer 350 undergoes a polishing and/or etching procedure.The polishing and/or etching procedure includes, e.g. achemical-mechanical planarization (CMP) (i.e., chemical-mechanicalpolishing) process that is used to polish the surface of the passivationlayer 350 and remove topographical irregularities.

FIG. 3I is a cross-sectional view of the semiconductor device includingsource and drain contacts 381, 391, 382, 392, which are formed throughthe first AlGaN layer 331, the second AlGaN layer 332 and thepassivation layer 350 and disposed on the first III-V semiconductormaterial layer 330 at one of the various stages of fabrication,according to some embodiments of the present disclosure. The source anddrain contacts may be formed as non-rectifying electrical junctions,i.e. ohmic contacts.

FIG. 3J is a cross-sectional view of the semiconductor device includinga mask 355, which is formed on the passivation layer 350 at one of thevarious stages of fabrication, according to some embodiments of thepresent disclosure. At this stage, the mask 355 has a pattern to exposeportions of the passivation layer 350 on top of the pGaN portion 341,342. As such, a first opening 357 is formed on the pGaN portion 341between the first pair of source 381 and drain 391 by etching thepassivation layer 350 with the patterned mask 355; a second opening 358is formed on the pGaN portion 342 between the second pair of source 382and drain 392 by etching the passivation layer 350 with the patternedmask 355.

FIG. 3K is a cross-sectional view of the semiconductor device includinga first gate 351 and a second gate 352, which are deposited and polishedin the first opening 357 and the second opening 358 respectively at oneof the various stages of fabrication, according to some embodiments ofthe present disclosure. According to various embodiments, the first gate351 and the second gate 352 may be formed of metal materials like:tungsten (W), nickel (Ni), titanium/tungsten/titanium-nitride (Ti/W/TiN)metal stack, or titanium/nickel/titanium-nitride (Ti/Ni/TiN) metalstack.

FIG. 3L is a cross-sectional view of the semiconductor device, where themask 355 is removed from the passivation layer 350 after the metal gatesare formed, at one of the various stages of fabrication, according tosome embodiments of the present disclosure. After the mask 355 isremoved, each of the source regions 381, 382, the drain regions 391,392, and the gate structures 351, 352 has an exposed portion on top ofthe passivation layer 350.

FIG. 3M is a cross-sectional view of the semiconductor device includingan interlayer dielectric (ILD) layer 360, which is formed on thepassivation layer 350, at one of the various stages of fabrication,according to some embodiments of the present disclosure. The ILD layer360 covers the passivation layer 350 and the exposed portions of thesource regions 381, 382, the drain regions 391, 392, and the gatestructures 351, 352 that are formed at the stage shown in FIG. 3L. TheILD layer 360 is formed of a dielectric material and may be patternedwith holes for metal interconnects or contacts for the source and draincontacts 381, 382, 391, 392 as well as the gate structures 351, 352.

FIG. 3N is a cross-sectional view of the semiconductor device includingmetal contacts 371, each of which is formed on a source or draincontact, at one of the various stages of fabrication, according to someembodiments of the present disclosure. As discussed above, the ILD layer360 is patterned with holes each of which is on one of the source anddrain contacts 381, 382, 391, 392. As such, the metal contacts 371 canbe formed in these holes to be in contact with the source and draincontacts 381, 382, 391, 392, respectively.

FIG. 3O is a cross-sectional view of the semiconductor device includinga first metal layer 372, which is formed on the metal contacts 371, atone of the various stages of fabrication, according to some embodimentsof the present disclosure. The first metal layer 372 includes metalmaterial and is formed over the ILD layer 360 and in contact with themetal contacts 371.

FIG. 4A and FIG. 4B show a flow chart illustrating an exemplary method400 for forming a semiconductor device including dual polarizationtransistors, in accordance with some embodiments of the presentdisclosure. As shown in FIG. 4A, at operation 402, a transition/bufferlayer is formed on a semiconductor substrate by epitaxial growth. A GaNlayer is formed at operation 404 on the transition/buffer layer byepitaxial growth. At operation 406, a first AlGaN layer is formed on theGaN layer by epitaxial growth. At operation 408, a second AlGaN layer isformed with a mask on the first AlGaN layer by epitaxial growth. Atoperation 410, the mask on the first AlGaN layer is removed. Atoperation 412, a p-type doped GaN layer is deposited and defined on thefirst and second AlGaN layers. At operation 414, a passivation layer isdeposited and polished on the p-type doped GaN layer and the AlGaNlayers. The process then goes to the operation 416 in FIG. 4B.

As shown in FIG. 4B, at operation 416, source and drain ohmic contactsare formed through the passivation layer and the AlGaN layers. Atoperation 418, openings are defined for metal gate areas on the p-typedoped GaN layer by etching with a mask. At operation 420, the metal gatematerial is deposited and polished in the openings to form the gates. Atoperation 422, the mask on the passivation layer is removed. Atoperation 424, a dielectric layer is deposited and polished on thesources, drains, gates and the passivation layer. Metal contacts areformed and defined at operation 426 on the sources, drains and gates. Atoperation 428, a first metal layer is formed and defined on thedielectric layer and the metal contacts. The order of the operationsshown in FIG. 4A and FIG. 4B may be changed according to differentembodiments of the present disclosure.

In an embodiment, a semiconductor structure is disclosed. Thesemiconductor structure includes: a substrate; an active layer that isformed over the substrate and comprises a first active portion having afirst thickness and a second active portion having a second thickness; afirst transistor comprising a first source region, a first drain region,and a first gate structure formed over the first active portion andbetween the first source region and the first drain region; and a secondtransistor comprising a second source region, a second drain region, anda second gate structure formed over the second active portion andbetween the second source region and the second drain region, whereinthe first thickness is different from the second thickness.

In another embodiment, a circuit is disclosed. The circuit includes afirst transistor including a first gate, a first source and a firstdrain; and a second transistor including a second gate, a second sourceand a second drain. The first transistor and the second transistor areformed on a same semiconductor wafer including an active layer thatcomprises a first active portion under the first gate and a secondactive portion under the second gate. The first active portion has afirst thickness. The second active portion has a second thicknessdifferent from the first thickness.

In yet another embodiment, a method for forming a semiconductorstructure is disclosed. The method includes: forming an active layerover a substrate, wherein the active layer comprises a first activeportion having a first thickness and a second active portion having asecond thickness; forming a first transistor comprising a first sourceregion, a first drain region, and a first gate structure formed over thefirst active portion and between the first source region and the firstdrain region; and forming a second transistor comprising a second sourceregion, a second drain region, and a second gate structure formed overthe second active portion and between the second source region and thesecond drain region, wherein the first thickness is different from thesecond thickness.

The foregoing outlines features of several embodiments so that thoseordinary skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodimentsintroduced herein. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A semiconductor structure, comprising: asubstrate; an active layer that is formed over the substrate andcomprises a first active portion having a first thickness and a secondactive portion having a second thickness; a first transistor formed overthe first active portion; and a second transistor formed over the secondactive portion, wherein the first thickness is different from the secondthickness so as to cause a first threshold voltage of the firsttransistor to be different from a second threshold voltage of the secondtransistor.
 2. The semiconductor structure of claim 1, wherein: thefirst transistor and the second transistor are high electron mobilitytransistors to be used in a same multi-stage driver circuit.
 3. Thesemiconductor structure of claim 1, wherein: the first transistorcomprises a first source region, a first drain region, and a first gatestructure formed over the first active portion and between the firstsource region and the first drain region; and the second transistorcomprises a second source region, a second drain region, and a secondgate structure formed over the second active portion and between thesecond source region and the second drain region .
 4. The semiconductorstructure of claim 3, wherein: the first active portion is thinner thanthe second active portion.
 5. The semiconductor structure of claim 4,wherein: the first thickness is smaller than 80% of the secondthickness.
 6. The semiconductor structure of claim 1, further comprisinga channel layer formed over the substrate and below the active layer,wherein: the channel layer comprises a first III-V semiconductormaterial; and the active layer comprises a second III-V semiconductormaterial that is different from the first III-V semiconductor material.7. The semiconductor structure of claim 6, wherein: the first III-Vsemiconductor material comprises gallium nitride (GaN); and the secondIII-V semiconductor material comprises aluminum gallium nitride (AlGaN).8. The semiconductor structure of claim 1, further comprising apolarization modulation layer that comprises a first polarizationmodulation portion disposed on the first active portion and a secondpolarization modulation portion disposed on the second active portion,wherein: the polarization modulation layer comprises p-type doped GaN;the first gate structure is disposed on the first polarizationmodulation portion; the second gate structure is disposed on the secondpolarization modulation portion; and the first polarization modulationportion has a thickness different from that of the second polarizationmodulation portion.
 9. A circuit, comprising: a first transistor; and asecond transistor, wherein: the first transistor and the secondtransistor are formed on a same semiconductor wafer including an activelayer that comprises a first active portion under at least a portion ofthe first transistor and a second active portion under at least aportion of the second transistor, the first active portion has a firstthickness, and the second active portion has a second thicknessdifferent from the first thickness.
 10. The circuit of claim 9, wherein:the first transistor has a first threshold voltage; and the secondtransistor has a second threshold voltage that is different from thefirst threshold voltage.
 11. The circuit of claim 9, wherein: the firsttransistor comprises a first gate, a first source and a first drain; andthe second transistor comprises a second gate, a second source and asecond drain, and wherein at least one of the first source and the firstdrain is electrically connected to a ground voltage; and at least one ofthe second source and the second drain is electrically connected to apositive supply voltage.
 12. The circuit of claim 11, wherein: the firstthreshold voltage is higher than the second threshold voltage.
 13. Thecircuit of claim 12, wherein: the first active portion is thinner thanthe second active portion.
 14. The circuit of claim 12, wherein: atleast one of the first source and the first drain is electricallyconnected to an output pin of the circuit.
 15. The circuit of claim 12,wherein the first transistor is at least one of: a high voltageenhancement-mode high electron mobility transistor (HV E-HEMT); a lowvoltage enhancement-mode high electron mobility transistor (LV E-HEMT);and a low voltage depletion-mode high electron mobility transistor (LVD-HEMT).
 16. The circuit of claim 12, wherein the second transistor is alow voltage enhancement-mode high electron mobility transistor (LVE-HEMT).
 17. The circuit of claim 11, wherein the first gate isphysically coupled to the second source.
 18. A method for forming asemiconductor structure, comprising: forming an active layer over asubstrate, wherein the active layer comprises a first active portionhaving a first thickness and a second active portion having a secondthickness; forming a first transistor over the first active portion; andforming a second transistor over the second active portion, wherein thefirst thickness is different from the second thickness.
 19. The methodof claim 18, wherein: the first transistor and the second transistor arehigh electron mobility transistors to be used in a same multi-stagedriver circuit; the first transistor has a first threshold voltage; thesecond transistor has a second threshold voltage that is lower than thefirst threshold voltage; and the first active portion is thinner thanthe second active portion.
 20. The method of claim 18, wherein formingthe active layer comprises: forming a first sub-layer over thesubstrate; forming a second sub-layer on the first sub-layer with a maskcovering part of the first sub-layer, wherein each of the firstsub-layer and the second sub-layer comprises aluminum gallium nitride(AlGaN); and removing the mask to form the first active portion and thesecond active portion that have different thicknesses.